Terminal size under OpenMPI

[mrzv]

Background information

What version of Open MPI are you using? (e.g., v3.0.5, v4.0.2, git branch name and hash, etc.)

v4.0.4

Describe how Open MPI was installed (e.g., from a source/distribution tarball, from a git clone, from an operating system distribution package, etc.)

Homebrew (same result for packages from the ArchLinux distribution)

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Details of the problem

When querying the size of the terminal under OpenMPI, the result comes back as 0 columns, which messes up code that relies on this number for formatting.

Here's an example in Python.

# terminal-size.py
import shutil
print(shutil.get_terminal_size())

Running without OpenMPI, reports 191 columns.

python3 terminal-size.py            
os.terminal_size(columns=191, lines=75)

Running with OpenMPI, reports 0 columns.

mpirun -n 1 python3 terminal-size.py
os.terminal_size(columns=0, lines=0)

FWIW, under MPICH the terminal size is reported as 80 columns.

[jsquyres]

This is probably not surprising. I don't remember exactly how Open MPI's pty code works (I'd have to rummage around / refresh my memory), but MPI processes are generally remote from mpirun, and therefore it's probably reasonable to report 0 rows/columns. I.e., even though we transport the stdout/stderr of each MPI process back to mpirun, it's not actually running in the same terminal session an mpirun.

In your example, I'm presuming that you're running locally, so an argument may be able to be made that if the MPI process has a simple ancestral relationship to mpirun, then we should report the same rows/columns as mpirun. I'm not offhand sure why we don't do that, but it could well be that we took a unification / simplification step and decided that all MPI processes report 0 rows/columns...?

Is this creating a problem? I.e., is there a use case where an MPI process should report >0 rows/columns?

[mrzv]

I'm not sure I agree with the reasoning why 0 columns is a reasonable thing to report. Remote or not it's normal for MPI applications to output something. Even when redirecting the output to a pipe, you normally get some reasonable default number of columns (usually 80). With the above example:

python3 terminal-size.py | cat
os.terminal_size(columns=80, lines=24)

The reason of course is that if you want to display any non-trivial formatting, it's useful to know the screen width. (That's how I've run into this problem.) MPICH reports the same 80x24, BTW.

Passing the true terminal size to all MPI processes would be a great solution (after all by default you are sending all of their output to the terminal), but the simple result that one gets for a pipe (80x24) would also work. I'm not hoping for anything fancy; it's just that 0 columns makes very little sense and breaks some code.